Essentially, Pi-hole becomes the local network DNS server where it parses all client lookup requests for "port 80". If the lookup is in an ad-domain and others, that particular lookup is blocked.

As I've already found, Pi-Hole works from the command line, in any case. The only question is getting to the GUI console for ease of configuration and statistics. To get to the admin console, Pi-hole will also work with web admin login page on another port (other than 80) but the errata states that changing from port 80 prevents full web page blocks and that some picture frame blocks will not work properly.

By editing a Pi-Hole config file - lighttpd.conf - to use another port, I can now login to the console with the following. -> omvserverip:8888/admin

In the bottom line, it works but functionality is reduced and the .conf file is over written in a version upgrade. (Much as OMV does in some cases.)
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In the bottom line, whether using an Ethernet sub-int and a secondary IP, or some other work around, I believe PI-Hole would make an excellent OMV plugin for reasons previously stated. Advertising is getting way out of hand. While I was testing Pi-Hole I was getting snappy performance from sites with a ton of advertisement. In my experience, this is a first, despite having a cable modem and a fast connection. Again, unless one "likes" being bombarded with unwanted ad's, accidental "one click" redirects, and poor performance from even the fastest connections, Pi-Hole seems to be indispensable.

Ports
We need ports 53, 80, and 4711. Port 80 is optional if you decide not to install the Web dashboard during installation.

Port 80 should be used by lighttpd
If you have another Web server already running, such as Apache, Pi-hole's Web server will not work. You can either disable the other Web server or change the port on which lighttpd listens, which allows you keep both Web servers running.

CHANGES SHOULD BE MADE IN A SEPERATE CONFIG FILE:
/etc/lighttpd/external.conf

Seems to be some browser ad technical reason as explained in their forum. An option would be to use docker with macvlan, however omv won't be able to use the pihole dns server as their is no host-container comm in that mode. Not to important as the filtering is for web browsing.

Port 80 should be used by lighttpdIf you have another Web server already running, such as Apache, Pi-hole's Web server will not work. You can either disable the other Web server or change the port on which lighttpd listens, which allows you keep both Web servers running.

This is the crux of the issue. Since OMV is running , a Web server is running, and disabling the Web console of a headless server (OMV) is not an option.
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(Realizing that the following is suggested by a non-coder:)
As noted before - Pi-Hole seems to work fine from the CLI but the Web console makes a lot of operations (custom blacklists, white listing, etc.,) a lot easier, much as it is when configuring OMV.
The real bonus in using Pi-Hole are the statistics. It's one thing to understand the nature of the flood of advertising and it's impact on your browser / PC. It's another thing to see it, and to be able to see it (the statistics), the console is needed.
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The console problem could solved altogether, without modifying any config files, "if" OMV can be made to operate solely from the primary ip address assigned to eth0: .

Pi-Hole will operate from a secondary address, assigned to a configured subinterface eth0:0. However, when the sub-int is configured, (in my tests) OMV takes control of the secondary address as well.

So, one question seems to be, can OMV be configured to use a single IP address (assigned to etho:), if a second IP address exists (assigned to sub-int eth0:0). Of course this but a single approach toward addressing the issue.
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In my case, I have Pi-Hole running well along side of OMV, for all the clients on my network. That's not the issue. On the other hand, I believe that if Pi-Hole can be integrated into OMV, in the form of a plugin, it may increase the user base. Being inundated with advertising to the point where a browser and PC slows down is, well, irritating to say the least. If a little add-on can knock all of that out, well, that would be a really nice feature to have.

Since OMV is running , a Web server is running, and disabling the Web console of a headless server (OMV) is not an option.

Login via SSH, stop nginx, install pi-hole, adjust server.port in lighthttpd's config, reboot. This is done in less time than writing the essay above and posting a mini tutorial takes also just a few minutes...

I looked into it a while back and it was too much work to make a plugin. Running it in a docker or on its own RPi seems to be a lot easier

I looked to see if a Docker was available for Pi-Hole and didn't find an image. And I think asking a new user to custom configure a container to run Pi-Hole would be beyond the skill set of most.
(Since I have yet to attempt something like that, I'd have to include myself in the "limited skill-set" group. )

From the post above, RE. a sub-interface on etho:
Can I take it's not possible to prevent OMV from automatically taking over a secondary IP address? While that wouldn't make it a plugin, that would solve the Web console problem. (I wonder if pseudo int's are used in Linux....)

Login via SSH, stop nginx, install pi-hole, adjust server.port in lighthttpd's config, reboot. This is done in less time than writing the essay above and posting a mini tutorial takes also just a few minutes...

And that comes from one who specializes in exhaustive essays. With your eminent skill set, I'm sure your input may be needed elsewhere....

Thanks for the pointer to the guide, but I have it working. On the other hand, if you actually read "the essay" it's not about me. It's about new users and expanding the OMV tent.

Here is the docker image but I wouldn't be much help configuring the docker either. Running it on its own rpi is probably the easiest.

Pi-Hole wasn't in the default list but a search for the image name you supplied found and pulled it, from within the docker plugin. Configuring a Docker container, for bridged networking , from within my test VM (which is also bridged) is proving to be something to work out. I'll do a test build on a real box and try again.

I learned something in this exercise: Linux supports both sub-int's and pseudo int's.

Thanks for the info and the history recap (that you looked at Pi-Hole before). It's kind of a shame that it's a PITA to code. I've been using it less than a day and can't imagine going back.
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BTW: It's nice to get a few lines from folks without, shall we say,, outsized ego's. What you do, and especially the way you present it, is sincerely appreciated.

Pi-Hole wasn't in the default list but a search for the image name you supplied found and pulled it, from within the docker plugin. Configuring a Docker container, for bridged networking , from within my test VM (which is also bridged) is proving to be something to work out. I'll do a test build on a real box and try again.

I learned something in this exercise: Linux supports both sub-int's and pseudo int's.

Thanks for the info and the history recap (that you looked at Pi-Hole before). It's kind of a shame that it's a PITA to code. I've been using it less than a day and can't imagine going back.

I'm sure if you wrote a guide to setting up the docker plugin to use this docker, it would be popular

I'm sure if you wrote a guide to setting up the docker plugin to use this docker, it would be popular

I now have a Docker Plugin/Pi-hole set up that works on OMV.

Unlike the lighttpd.conf server port workaround - with Docker it's possible to assign an IP address to a container that's separate from the OMV server. With that address and direct access to port 80 and others, Pi-hole is fully functional.

I did some preliminary testing on 3 separate x64 hardware platforms. The Pi-Hole container, with internal logs and configuration data is writable and persistent (survives a reboot), but I believe it needs a bit more testing. A "How To" guide is in the works.

Unlike the lighttpd.conf server port workaround - with Docker it's possible to assign an IP address to a container that's separate from the OMV server. With that address and direct access to port 80 and others, Pi-hole is fully functional

If you used macvlan, make sure omv queries directly to the fw or another upstream dns not the pihole.

What you see in the bintray is not a plugin. it is the borgbackup package itself. I have been developing the plugin against the 1.1.x branch instead of the 1.0.x branch that is in Debian stretch.

I am using the plugin now and it works quite well for me. There is no documentation and no one else has tested. I may still add features too. Just haven’t had much time to do more on it. I can see about putting it in a place to download if you are interested in testing.